1. Field of the Invention
The present invention relates to circuits which apply a voltage across electrodes of electro-optic modulators to change the intensity of light at a photosensitive surface.
2. Description of the Prior Art
Electro-optic materials are those whose optical properties change in accordance with the strength of an electric field established within them. These materials make possible an electrically controlled "electro-optic modulator." In this disclosure, the term "modulator" includes a device which changes the intensity of light in response to an applied electric signal. An "electro-optic modulator" includes a member formed of electro-optic material which receives plane polarized (linear) light and which changes the state of polarization of such light in response to an established electric field. An analyzer receives light from the member and blocks that light whose plane of polarization has not changed (no established electric field) while transmitting light when its plane of polarization has been changed by an established electric field. By changing the electric field in the electro-optic member, light which passes through the analyzer is modulated.
One example of an electro-optic material used in modulators is lanthanum-doped lead zirconate titanate (PLZT). Although PLZT is a preferred electro-optic material, it will be recognized by those skilled in the art that other electro-optic materials can also be used to change the polarization of light.
Without an electric field being established, some compositions of PLZT are optically isotropic, while others exhibit a static birefringence. In either case, when a voltage is applied across electrodes and an electric field is established through a member made of PLZT, the PLZT crystal structure changes. This change in crystal structure causes a change in birefringence. An optic axis is thereby formed which is aligned parallel to the electric field lines. The optic axis is a direction and not just one particular line. The applied voltage produces the electric field. The intensity of the field causes the polarization of light to rotate. Thus, the intensity of light which passes through a modulator is a function of the applied voltage.
Problems exist with such PLZT electro-optic modulators. Changes in output light can occur with a fixed voltage applied across the electrodes. Also the intensity of light at a given voltage is often dependent on whether the applied voltage was increased or decreased to achieve that level. This is known as a hysteresis effect.
Because of this hysteresis effect, commonly assigned U.S. Pat. No. 4,631,551, issued Dec. 23, 1986 to Vergona discloses an apparatus where the light intensity at the output of the modulator is compared with the desired level to decide if changes should be made in the voltage applied across the electrodes of the modulator. This arrangement includes a feedback arrangement which produces a voltage signal which is a function of the difference in intensity of the light beam from a desired level. This voltage signal drives a circuit which changes the voltage applied across the modulator electrodes. The voltage applied (V.sub.applied) across the electrodes should have a relatively wide voltage range to achieve a desired gray scale. Since this voltage at the high end of this range can be in the order of several hundred volts, a high current expensive power supply maybe required. Another important consideration is to provide the circuit with adequate bandwidth to reduce distortions that can appear in V.sub.applied.
The object of this invention is to provide a circuit for adjusting the voltage applied across an electro-optic modulator electrodes which does not draw an excess current while still providing an adequate bandwidth for the exposure of high quality images.